The present invention relates to a method of operating a blood treatment device for determining the blood flow in a vascular access during an extracorporeal blood treatment, and to a device for determining the blood flow in a vascular access during an extracorporeal blood treatment.
In the methods used for chronic blood purification therapy such as hemodialysis, hemofiltration and hemodiafiltration, blood is sent through an extracorporeal circulation loop in a blood treatment unit, such as a dialyzer or filter. To serve as the access to the blood vessel system, an arteriovenous fistula is often created surgically and is then generally punctured with an arterial needle and venous needle (double needle dialysis). Likewise, use of a vascular implant (PTE graft) is also possible. The term xe2x80x9cvascular accessxe2x80x9d, as it is used below, is understood to refer to any type of access to a patient""s blood vessel, but in particular to the connection between a patient""s artery and vein.
Typical flows within a satisfactorily functioning vascular access are in the range of 1100 mL/min. Measurement of blood flow and vascular pressure is of crucial importance to monitoring the functioning of the access. Vascular implants showing a flow rate of less than 600 to 800 mL/min or an abnormal pressure are associated with a much higher risk of thrombosis. A thrombosis develops as a result of an unknown stenosis which leads to a reduction in blood flow in the vascular access. Through early detection of vascular accesses with a reduced blood flow, it is therefore possible to prevent imminent thromboses. In addition, by identifying vascular accesses with pathologically elevated flow rates above 2000 mL/min, overloading of the patient""s cardiovascular system is prevented.
German Patent application 4 024 434 A1 describes a device for ultrafiltration monitoring in blood purification processes, having a pressure measurement device arranged in the extracorporeal blood circulation and an analyzer unit where the measured pressure values are stored in chronological order, and where a change in blood viscosity is deduced from a change in pressure values.
A device for measuring the flow through a fistula is described in German Patent 19 541 783 C1. This measurement of fistula flow is based on measuring the temperature in the arterial branch of the extracorporeal circulation while there occur variations in extracorporeal blood flow.
Another method of determining the blood flow in a vascular access is based on a measurement of recirculation before and after exchanging the arterial and venous blood tubes on the needles. This method yields good clinical results, but has the disadvantage that when the tubes are improperly exchanged, there is a risk of blood loss and infections, plus a residual pulmonary embolism risk.
In everyday clinical practice, the static pressure in the vascular access is measured after turning off the blood pump and the ultrafiltration unit. However, when the blood pump is stopped there is the risk of coagulation in the blood tubing system.
The present invention provides a method of operating a blood treatment device that makes it possible to determine the blood flow in the vascular system with high reliability, without any technical expense and without requiring that the blood tube connections be exchanged. This invention also provides a device that is relatively simple to implement technically, so that the blood flow in the vascular access can be determined with a high certainty without requiring that the blood tube connections be exchanged.
In one aspect, the invention is a method of operating a blood treatment machine for determining the blood flow QB in a vascular access of an extracorporeal circulation during an extracorporeal blood treatment, the blood entering a blood treatment unit of the blood treatment machine through an arterial connection of an arterial branch in fluid connection with the vascular access, and is returned through a venous connection of a venous branch in fluid connection with the vascular access. The method comprises the steps of measuring pressures Part, Pven, Part comp, Pven comp in one of the arterial or venous branch while the vascular access is open and blood flows through said vascular access between the arterial and venous connections, measuring the pressures while the vascular access is interrupted and no blood flows through the vascular access between the arterial and venous connections, varying blood flow QB in the extracorporeal circulation, and determining the blood flow QF in the open vascular access between the arterial and venous connections from the measured values of the pressures Part, Pven, Part comp, Pven comp.
In another aspect, the invention is a device for determining blood flow in a vascular access during an extracorporeal blood treatment, comprising an arterial branch of an extracorporeal circulation in fluid connection with the vascular access at an arterial connection, a blood treatment unit for receiving blood from the arterial branch, a venous branch of the extracorporeal circulation in fluid connection with the vascular access at a venous connection, and a blood pump connected to the extracorporeal circulation. The device also includes a control unit for varying the flow rate of the blood pump, at least one of an arterial and a venous measurement device for measuring pressures Part, Pven, Part comp, Pven comp respectively in the arterial and venous branch of the extracorporeal circulation with the vascular access open and with the vascular access interrupted, means for varying a blood flow QB in the extracorporeal circulation, a memory unit for storing the measured arterial and venous pressure, and a computer unit adapted to determine the blood flow QB in the open vascular access from the measured values of the arterial and venous pressure Part, Pven, Part comp, Pven comp.
In the method according to this present invention, the blood flow in the vascular access is determined by performing a pressure measurement with an open vascular access while blood flows through the vascular access between the arterial and venous connections. The method also uses an interrupted vascular access while no blood flows through the same, while the blood flow in the extracorporeal circulation is varied. The blood flow in the open vascular access can then be determined from the measured values of the pressure in the vascular access that is open and interrupted. The blood flow can be determined either exclusively from the measured values for the pressure in the arterial branch, with the vascular access open and interrupted, or exclusively from the values for the pressure in the venous branch, with the vascular access open and interrupted.
However, it is also possible to use both the arterial pressure and the venous pressure values with both open and interrupted vascular access to determine the blood flow. The vascular access is preferably pressed by hand between the needles, which offers advantages in practice. This procedure can also be carried out with a compensation tube, a cuff or other similar device.
All measured values can be determined first with the vascular access open or interrupted, and only then are all the measured values determined with the vascular access in the other one of the interrupted or open condition, respectively, while the blood flow is varied within predetermined limits. The measured values can be recorded in two successive cycles, one with the vascular access open and one with it interrupted.
In a first variant of the claimed method, the blood flow can be determined in the extracorporeal circulation where the pressure in the arterial or venous branch, with the vascular access interrupted, is equal to the pressure in the arterial or venous branch, respectively, with the vascular access open. This step determines the blood flow in the open vascular access. In this case, the extracorporeal blood flow is equal to the blood flow in the vascular access. It is advantageous that only one pressure measurement, either in the arterial branch or in the venous branch, is necessary.
A second variant of this method provides for a measurement to be performed in the arterial and venous branches, to determine the blood flow in the extracorporeal circulation at which the pressure in the arterial branch having the vascular access interrupted is equal to the arterial pressure having the vascular access open. This method also determines the extracorporeal blood flow at which the venous pressure with the vascular access interrupted is equal to the venous pressure having the vascular access open. The blood flow in the open access is then advantageously determined by forming the average of these two extracorporeal blood flow values.
In another variant of this method, the difference between the pressure in the arterial branch having the vascular access interrupted and the pressure in the arterial branch having the vascular access open, and the difference between the venous pressure having the vascular access interrupted and the venous pressure having the vascular access open are determined as a function of the extracorporeal blood flow. For the case when the two differences are equal to zero, the extracorporeal blood flow is equal to the blood flow in the open vascular access.
The measured pressure values can be preferably stored in memory in chronological order. The parameters of a function representing the arterial and/or venous pressure as a function of the extracorporeal blood flow can be advantageously determined from the discrete measured values. Known mathematical procedures can be used to this end. Depending on the required accuracy, a larger or smaller number of measured values may be necessary. Outside of the defined limits, the arterial and/or venous pressure is advantageously determined by extrapolation of the function curve, so the blood flow need be varied only within relatively narrow limits.
The arterial and/or venous static pressure in the vascular access can also be determined from the measured values for the arterial and/or venous pressure in the extracorporeal circulation. To do so, by extrapolating the function curve, the arterial and/or venous extracorporeal pressure at which the extracorporeal blood flow is equal to zero is determined.
The invention described here includes an arterial and/or venous pressure measurement device for measuring the pressure in the respective arterial and/or venous branches. A control unit can be provided to permit variation of the extracorporeal blood flow by varying the flow rate of the blood pump installed in the extracorporeal circulation. The measured values can be stored in a memory unit. The blood flow in the vascular access can be calculated in a computer unit on the basis of the stored measured values.